The present invention generally relates to a process for plasma treatment of particulate matter, and more particularly, to an apparatus for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner, and a process for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner.
Thin film technologies are widely used to modify surface properties such as wettability, hardness, abrasion, adhesion, permeability, refractive index and biocompatibility of particles. Plasma enhanced chemical vapor deposition (PECVD) is one example of a particularly useful technique for modifying such surface properties. As PECVD operates under vacuum, it is possible to lower the deposition temperature and thereby to improve the quality of the coating. This is especially the case for coating of temperature sensitive powders. If a suitable organic vapor (monomer) is introduced into a plasma, or when a plasma of an organic vapor is created, polymerization of the vapor occurs and a polymeric film is formed, which can be deposited to a temperature sensitive powder. Energetic species from the plasma such as ions, atoms, metastables, as well as electrons and a broad electromagnetic spectrum, dissociate or modify the gaseous monomers to form precursors, which chemically react and lead to the desired film formation. An electrical field is applied that accelerates the free electrons in the discharge, which then deliver energy to the atoms or molecules through collisions. Typically the degree of ionization is less than 0.1%. This allows gas temperatures to be kept below 100xc2x0 C. and treatment of heat sensitive powders. Hence the PECVD method provides a means of surface coating since such a polymer deposition can be highly crosslinked and strongly bonded to the surface.
The surface characteristics of powders or pigments is a very important factor when powders are used in many industrial applications. Powders including carbon black, zinc oxide, titanium oxide, pigment, silica, mica and zeolite are useful raw materials in rubber, electronic, paint and petrochemical industries. The surface properties influence the flowability, dispersability, solubility and adhesive properties of powders. In many applications it is necessary to alter the surface characteristics of the powders from hydrophobic to hydrophilic or inversely from hydrophilic to hydrophobic without changing the bulk properties. Plasma polymerization techniques offer the opportunity to deliver the powders an uniform, ultrathin and pinhole free coating and consequently, many applications have surfaced in the past few years.
Several prior art studies have reported work on cold plasma treatment of fillers (mica, silica) used in engineering plastics. In one study, a Ar-C3H6 plasma was used to modify the surface of calcium carbonate powder and the immersion of heat measurement was used as the evaluation method of the surface modification effect. Another study has shown that it has been possible to increase the dispersion rate and dispersion stability of NH3xe2x80x94 and O2-plasma treated pigments in water soluble acrylic resin systems. Yet another study has disclosed the use of low-temperature gas plasma treatment of waxes in a rotating drum reactor, and it has been found that increasing their wettability related to improved dispersability, emulsifiability, solubility and reactivity towards a wide variety of materials.
It is known that plasma fluidized bed reactors can provide intimate mixing between the powders and the reactive gas to improve both the reaction rate and the uniformity of the treated surface. Powder is placed on the porous plate in the reactor, which is positioned vertically, and a gas is injected from the gas inlet at the bottom of the reactor. The gas passes up through a bed of the powder. At more than a critical flow rate of the gas stream the pressure drops and the drag on individual powder increases. As result, the powders start to move and become suspended in the fluid. This state is called xe2x80x9cfluidizationxe2x80x9d and means the condition of fully suspended particles. In certain cases, a vibrator can be connected to the reactor for maintaining the powder in the jiggling fluidized-bed state.
When a particle is traveling through a plasma containing an organic monomer, a film of a plasma-polymerized organic material is deposited even on high surface area ( greater than 100 m2/g) particles. The difficulty in this process, which has thus far impeded its large-scale utilization, is the lack of good contact between the substrate and the plasma. In contrast to flat surfaces there is still no satisfactory process for coating powders and granules. Conventional reactors, such as barrel- or jar-type, cannot be used for powder materials due to the lack of solid mixing. Thus, the inventors of the present invention have conducted a variety of theoretical and experimental studies to improve the plasma-particle reactions.
It is difficult to do plasma treatment of powders because of aggregation and large surface area per unit of mass of the powder. The stability of plasmas in fluidized beds can be negatively affected by interactions of particles with the plasma. Particles may directly absorb radio frequency or microwave energy or they could collide with excited gas species, namely the electrons and reduce the electron density. When the solids concentration gets too high, large parts of the plasma volume can be affected and as a consequence the plasma could extinguish. It is known that the presence of a radio frequency glow discharge greatly reduces the agglomeration of particles in the fluidized-fed preparation of calcium superoxide from calcium peroxide diperoxyhydrate. The reduced agglomeration is possibly due to the reduction of static charges. However, in order to prepare powders in a fluidized reactor, research not only on chemical reactions but also on powder handling becomes critical. While the particles handled in conventional fluidized beds are mostly larger than 10-30 microns, it is precisely the plasma treatment of micron-sized particles that remains a challenge.
Thus it has been desirable to conduct further research on efficient reactor design and reactor modeling. It has been extremely desirable to have a new and improved process for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner, because conventional models for fluidization reactors seem not to be applicable to plasma reactors.
The following invention is directed to overcome one or more problems, as set forth above.
The present invention discloses an apparatus for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner, and a process for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner.
In one aspect of the present invention, an apparatus for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner includes a vacuum processing chamber which comprises a first hopper section, a longitudinal middle section, and a second hopper section. The first hopper section has an inlet, a closed end, a longitudinal middle portion and an open end. The first hopper section is oriented along a first longitudinal axis. The inlet of the first hopper section is removably connectable to a particulate matter feed valve. The longitudinal middle section has a first open end, a longitudinal middle portion, and a second open end. The longitudinal middle section is oriented along the first longitudinal axis. The vacuum processing chamber is rotatable at the mid-point of the longitudinal middle section through at least 180 degrees about a second longitudinal axis. The second longitudinal axis is perpendicular to the first longitudinal axis. The second hopper section has an inlet, a closed end, a longitudinal middle portion and an open end. The second hopper section is oriented along the first longitudinal axis, and the inlet of the second hopper section is removably connectable to a polymerizable liquid monomer feed valve. The open end of the first hopper section is connected with the first open end of the longitudinal middle section through a first valve, and the open end of the second hopper section is connected with the second open end of the longitudinal middle section through a second valve. The particulate matter feed valve is removably connected to the inlet of the first hopper section and the polymerizable liquid monomer feed valve is removably connected to the inlet of the second hopper section when the vacuum processing chamber is at a rotational position xe2x80x9caxe2x80x9d. The particulate matter feed valve is removably connected to the inlet of the second hopper section and the polymerizable liquid monomer feed valve is removably connected to the inlet of the first hopper section when the vacuum processing chamber is at a rotational position xe2x80x9cbxe2x80x9d. Further, the rotational position xe2x80x9cbxe2x80x9d is 180 degrees with respect to the rotational position xe2x80x9caxe2x80x9d. The first hopper section is adapted to deliver untreated and plasma treated particulate matter to the longitudinal middle section and the second hopper section is adapted to receive one or more of plasma treated particulate matter from the longitudinal middle section and the polymerizable liquid monomer when the vacuum processing chamber is at rotational position xe2x80x9caxe2x80x9d. The second hopper section is adapted to deliver untreated and plasma treated particulate matter to the longitudinal middle section and the first hopper section is adapted to receive one or more of the plasma treated particulate matter from the longitudinal middle section and the polymerizable liquid monomer when the vacuum processing chamber is at rotational position xe2x80x9cbxe2x80x9d. The longitudinal middle section is connected to a vacuum generation pump means for maintaining an vacuum or near-atmospheric pressure therein. The longitudinal middle section has plasma generating electrodes disposed therein for generating a plasma glow discharge by using a non-polymerizable plasma gas. The particulate matter is exposed to the plasma glow discharge as the particulate matter descends through the longitudinal middle section under gravity.
In another aspect of the present invention, a process for plasma induced graft polymerization of particulate matter in a continuous or semi-continuous manner includes the steps of providing a vacuum processing chamber which comprises a first hopper section, a longitudinal middle section, and a second hopper section. The first hopper section has an inlet, a closed end, a longitudinal middle portion and an open end. The first hopper section is oriented along a first longitudinal axis. The inlet of the first hopper section is removably connectable to a particulate matter feed valve. The longitudinal middle section has a first open end, a longitudinal middle portion, and a second open end. The longitudinal middle section is oriented along the first longitudinal axis. The vacuum processing chamber is rotatable at the mid-point of the longitudinal middle section through at least 180 degrees about a second longitudinal axis. The second longitudinal axis is perpendicular to the first longitudinal axis. The second hopper section has an inlet, a closed end, a longitudinal middle portion and an open end. The second hopper section is oriented along the first longitudinal axis, and the inlet of the second hopper section is removably connectable to a polymerizable liquid monomer feed valve. The open end of the first hopper section is connected with the first open end of the longitudinal middle section through a first valve, and the open end of the second hopper section is connected with the second open end of the longitudinal middle section through a second valve. The particulate matter feed valve is removably connected to the inlet of the first hopper section and the polymerizable liquid monomer feed valve is removably connected to the inlet of the second hopper section when the vacuum processing chamber is at a rotational position xe2x80x9caxe2x80x9d. The particulate matter feed valve is removably connected to the inlet of the second hopper section and the polymerizable liquid monomer feed valve is removably connected to the inlet of the first hopper section when the vacuum processing chamber is at a rotational position xe2x80x9cbxe2x80x9d. Further, the rotational position xe2x80x9cbxe2x80x9d is 180 degrees with respect to the rotational position xe2x80x9caxe2x80x9d. The process further includes the steps of rotating the vacuum processing chamber about the second longitudinal axis such that the vacuum processing chamber is at a rotational position xe2x80x9caxe2x80x9d. The process further includes providing particulate matter into the first hopper section through the particulate matter feed valve, providing a vacuum of at least 500 mTorr in the longitudinal middle section and creating a plasma glow discharge in the longitudinal middle portion by the non-polymerizable gas to form a plasma zone. The process still further includes dropping the particulate matter from one of (i) first hopper section and (ii) second hopper section, into the plasma zone in the longitudinal middle section, and treating the particulate matter in the plasma zone as the particulate matter descends through the longitudinal middle section. Thereafter, the process includes collecting the plasma treated particulate matter in one of the first hopper section and the second hopper section, and rotating the vacuum processing chamber about the second longitudinal axis such that the vacuum processing chamber is at a rotational position xe2x80x9cbxe2x80x9d. The steps of dropping, treating, and collecting the particulate matter and rotating the vacuum chamber are repeated for a number of times sufficient to obtain a total residence time in a range of from about 0.001 seconds to 60 seconds. The process then includes providing the polymerizable liquid monomer into one of the first hopper section and the second hopper section, through the polymerizable liquid monomer valve and finally, exposing treated particulate matter to the polymerizable liquid monomer.